1. Field of the Invention
The invention relates to the estimation of the speed relative to the ground of a rotary wing machine with a view to automatic control of the hovering phase of flight. It pertains, more particularly to the availability of such an estimation.
2. Description of the Related Art
On a rotary wing machine, the control of the speed relative to the ground in a flight phase of approach to a phase of hovering flight, then in a phase of hovering flight, is performed using information regarding speed relative to the ground as soon as the air speed goes below a certain value, typically 60 knots. In the hovering flight phase, the concept of speed relative to the ground is essential, since it is the zeroing thereof in the three axes of the carrier which allows immobilization of the machine independently of the wind conditions.
Aboard a craft, the information on the speed vector relative to the ground may originate from various sources implementing different physical principles:                an inertial navigation platform providing accelerometric information and angular rates, as well as, depending on its degree of complexity, information regarding attitude, ground and vertical speeds referenced along the axes of the carrier, and information regarding heading followed and instantaneous position,        a Doppler radar delivering measurements of longitudinal and transverse ground speed in the axes of the carrier or        a satellite-based positioning receiver delivering measurements of ground speed along north and east geographical axes.        
An inertial platform bears various names: IMU, IRS, AHRS, INS (acronyms standing for the expressions: “Inertial Measurement Unit, Inertial Reference System, Attitude and Heading Reference System, Inertial Navigation System”) which are dependent on its sophistication and on the diversity of the information provided. Its cost, which depends on its performance, on the precision of its information and on the drifting of the inertial and gyroscopic sensors used, spans a wide range of prices, so that any even slightly advanced flying machine is equipped with one. It has the advantage of not calling upon any outside assistance, of being able to be very precise over the short term and of always being available. However these advantages are counterbalanced by measurement bias and noise due to its inertial sensors and by medium- and long-term drifting, inherent in the mode of obtaining the speed and attitude information, by integration of measurements of acceleration or of angular rates, and in the mode of obtaining the position information by double integration of acceleration measurements.
Doppler radar, which also does not call upon any outside assistance, delivers speed information free of medium- and long-term drifting since the Doppler effect is manifested at the speed level. It is on the other hand prone to disturbances induced by abnormal “lock-ons” which sometimes cause its information not to be available.
The satellite-based positioning receiver calls upon a constellation of positioning satellites, the positions of the visible satellites of which it knows by virtue of ephemerides, so as to determine, by triangulation, its position and hence that of the carrier craft. The speed information which it delivers is not affected by drift but it is noisy on account of the low power of the signals that it receives from the positioning satellites and of variable precision depending on the geometry of the triangulation determined by the positions of the visible satellites. Moreover, its information is not always available since it is prone to antenna masking preventing it from sensing a sufficient number of satellites of the positioning system to get a position fix, either on account of the attitude of the flying machine on which it is mounted, or on account of the relief when the flying machine is operating close to the ground.
Although they are affected by drift, inertial platforms are the most widely used equipment for determining speed and attitudes since they are the only ones to offer absolute continuity of service. To combat their drift, it has always been sought to periodically readjust the information that they deliver with the aid of positioning or speed measurement devices offering lesser continuity of service but devoid of drift, such as Doppler radars and satellite-based positioning receivers.
Various methods for readjustment or hybridization have been proposed. Some of them call upon a so-called “Kalman” filtering technique which consists in modelling the dynamic behavior of the various errors encountered and of their dependency relations with the signals through which they are perceived from outside. This leads to adaptive filters which are lengthy and expensive to fine-tune on account of the difficulties of the modelling and which demand not inconsiderable calculation power in order to operate.
Another known method for readjustment or hybridization, less effective than Kalman filtering which is more robust to the modelling of errors, but, on the other hand, less demanding calculationally, calls upon an estimator filter having the structure of a Kalman filter with fixed gains.